close all;
clc;
clear;
N=90;
Na=30;
R=0.8;
xd=[0,0];
yd=[1,4];
zd=[1,1];
set(gcf,'Position',[7 266 560 420]);
plot3(xd,yd,zd,'k');
axis([0,3,0,5,0,3]);
axis equal;
hold on;
xn(1)=0;
yn(1)=4;
zn(1)=1+R;
dy=3/N;
dA=pi*2/Na;
plot3([xn(1),xd(1)],[yn(1),yd(2)],[zn(1),zd(1)],'k');
view([-49,32]);
A=0;
C=[0.1,0.3,0.9];

zjliu2001@163.com
rand('state',1)
Cr=rand(Na,3);
for k=1:N;
    A=A-dA;
    xn(k+1)=R*sin(A);
    yn(k+1)=yn(k)-dy;
    zn(k+1)=1+R*cos(A);
    plot3([xn(k+1),xd(1)],[yn(k+1),yn(k+1)],[zn(k+1),zd(1)],'k');
    plot3([xn(k+1),xn(k)],[yn(k+1),yn(k)],[zn(k+1),zn(k)],'k');
    F=fill3([xn(k),xn(k+1),xn(1),xn(1)],...
            [yn(k),yn(k+1),yn(k+1),yn(k)],...
            [zn(k),zn(k+1),zd(1),zd(1)],Cr(mod(k-1,Na)+1,:),...
            'EdgeColor',Cr(mod(k-1,Na)+1,:));
end
text(0,3,2.2,'{\ithelix}','Fontsize',18,...
    'Fontname','Times new roman');
axis off;